Ofdm pre-equalizing

ABSTRACT

For example in case of an OFDM multicarrier transmission system the transmission characteristics of subcarriers of a multicarrier transmission system using a plurality of antenna elements ( 3, 3′ ) can be adjusted. Particularly the power and the phase of the subcarriers can be adapted. To this object the subcarrier frequency channel ( 2, 2′ ) characteristics of the multicarrier transmission are detected ( 11, 11′ ) at the side of the transmitter ( 3 ). The power of each subcarrier is then distributed by a weighting unit ( 14, 14′ ). The subcarriers can be further pre-equalized ( 1, 1′ ) by dividing the subcarrier signal respectively by the sum of the squared magnitude of the frequency channel characteristics of all subcarrier signals or a frequency characteristic of the selected antenna element ( 3, 3′ ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/586,481, filed Dec. 30, 2014, which is a continuation of 14/041,998,filed Sep. 30, 2013 (now U.S. Pat. No. 8,948,302, issued Feb. 3, 2015)which is a continuation of 13/892,798, filed May 13, 2013 (now U.S. Pat.No. 9,008,224, issued Apr. 14, 2015) which is a continuation of U.S.patent application Ser. No. 10 12/108,552, filed Apr. 24, 2008 (now U.S.Pat. No. 8,457,250, issued Jun. 4, 2013) which is a continuation of U.S.patent application Ser. No. 09/988,417, filed Nov. 16, 2001 (now U.S.Pat. No. 7,388,928, issued Jun. 17, 2008). This application further isbased upon and claims the benefit of priority from the European PatentApplication No. 00 125 436.6 filed Nov. 20, 2000. The entire contents ofeach of the above-listed documents are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for adjusting the transmissioncharacteristics 10 of subcarriers of a multicarrier transmission systemusing a plurality of antenna elements, to a computer software programproduct for implementing such a method when run on a computing device ofa transmitting device, a transmission diversity device as well as to abase station comprising such a transmission diversity device.

BACKGROUND ART

To reduce interference from other transmitters, the transmission powershould be as small as possible in any wireless transmission system. Thebackground of the present invention is the reduction of transmissionpower in multicarrier systems and particularly the reduction oftransmission power of OFDM wireless (LAN) systems. According to thesesystems a plurality of antenna elements share the transmission power.The transmission power of each subcarrier of the multicarriertransmission system can be adjusted such that the receiver can get aflat spectrum. In case sub- carriers are fading at all the antennaelements, these subcarriers should be transmitted with less power.

From EP 0 932 285 A2 a technique for the reduction of instantaneousmaximum power in multicarrier signals is known. According to thistechnique from a serial-to-parallel converting part, complex informationsubcarrier signals based on an information bit sequence are outputted toa fast inverse Fourier transforming part. The fast inverse Fouriertransforming part gives fast inverse Fourier transformation to inputtedsub-carrier signals to obtain a complex baseband time waveform of OFDMsymbols. A complex redundant subcarrier signal generating part generatescomplex redundant sub-carrier signals to reduce amplitude of a complexbaseband time waveform of OFDM symbols, and makes them undergo fastinverse Fourier transformation together with complex informationsubcarrier signals. With these processes, the amplitude of a complexbaseband time waveform of OFDM symbols is reduced and instantaneousmaximum power is also reduced.

From WO 97/26743 a multicarrier communication system and method for peakpower control is known. According to this technique a communicationdevice for simultaneously transmitting information on multiplesub-channels encodes information for each of the multiple sub-channelswith a coding scheme to produce channel encoded information. A maskvector derived from a redundancy in the coding scheme encodes thechannel encoded information to transform the channel encoded informationinto codewords having pairwise Euclidean distance properties identicalto those of the channel encoded information. Modulation of thesub-channels in accordance with the codewords in a modulators thenproduces a composite signal envelope having a peak-to-mean envelopepower ratio reduced relative to a power ratio for correspondinglymodulated channel encoded information.

From EP-0 881 782 A2 a single carrier maximum-ration synthetictransmission diversity device as shown in FIG. 8 is known. According tothis known transmission diversity device antenna elements are arrangedat intervals greater than J2. A signal received by an antenna element issent by way of an antenna multiplexer to a receiver, where the signal isdemodulated. The thus-demodulated signal is sent to a phase- and powerdetection section, where a phase and power of the signal are detected.On the basis of the result of such detection, a control sectioncalculated the phase and power of a transmission signal. On the basis ofthe result of the calculation, a transmission signal generation circuittransmits a transmission signal to each of the antenna elements by wayof the antenna multiplexer.

Note that the technique of EP 0 881 782 A2 claiming the calculation ofthe phase of a signal of each antenna cannot be applied to themulticarrier case, but only to a single carrier case, as it isimpossible to measure phases of received signals if there are more thantwo carriers.

In single carrier applications the phase of the signal changesfrequently as the symbols are transmitted serially. Therefore it isdifficult to compare phases between different antennas, as the phase isnot varying uniformly. Therefore in singles carrier applications a phasecomparison is preferably done using pilot symbols which phases arevarying uniformly or which are known.

From U.S. Pat. No. 5,973,642 adaptive antenna arrays for orthogonalfrequency division multiplexing systems (OFDM systems) with co-channelinterference is known. According to this known technique parameters foradaptive antenna arrays in OFDM systems with co-channel interference areestimated. The channel parameter estimation is performed using a twopass process that advantageously expands the temporal scope andconsiders past, present and future temporal channel estimations duringparameter estimation. Channel parameters are estimated by processing thesignals through fast Fourier transforms, temporal filters and inversefast Fourier transforms. The temporal filters optimize parametersestimation based upon instantaneous correlation of the received signals.This all takes place on the receiver's side of the OFDM system.

The technique of transmission antenna selection of OFDM subcarrier usingpower measurement of received subcarriers in a TDD system is known ofthe Japanese patent 1 1(1999)-205205 of NTT Docomo.

High speed radio systems use a very wide radio bandwidth. Therefore, thechannel characteristics cannot be flat for all subcarriers over the wideradio band even in short delay environment. Therefore, usually it is notpossible to use a complicated modulation scheme such as 16 or 64 QAM forall subcarriers. On the other hand, any radio communication systemsuffers from interference from other communication transmitters.

Principally a subcarrier which is in fading can be transmitted withstronger power. This solves the bit error problems, but causesinterference with other receivers and therefore reduces the totalcapacity of the system.

In view of the above-captioned prior art and problems it is the objectof the present invention to propose a technique which is particularlyadapted for lowering the transmission power in a multicarrier systemsuch as the OFDM system without decreasing the overall bit rate.

This object is achieved by means of the features of the independentclaims. The 5 dependent claims develop further the central idea of thepresent invention.

According to a first aspect of the present invention therefore a methodfor adjusting the transmission characteristics of subcarriers of amulticarrier transmission system using a plurality of antenna elementsis proposed. On the transmitting side the subcarrier frequency channelcharacteristics of the multicarrier transmission is detected. Then thepower of each subcarrier is distributed to the antenna elements bymultiplying (weighting) it (by) the complex conjugate of thecorresponding subcarrier frequency channel or [0, 11 value according tothe decision which antenna is selected for the corresponding subcarrierto make the communication more efficient. Therefore depending on thedetected frequency channel characteristics at each antenna element foreach subcarrier signal the antenna element having the best channelcharacteristics for said subcarrier signal can be used for transmission.

The power of the subcarriers can furthermore be pre-equalized on thetransmitter side by dividing the sub-carrier signals respectively by thesum of the squared magnitude of the frequency channel characteristics ofall antenna elements or by the magnitude of the frequency channelcharacteristics of the selected antenna.

Along with the pre-equalizing furthermore the phase of the subcarriersignals can be respectively compensated for at the transmission sideaccording to the detected frequency channel characteristics.

Alternatively or additionally, the phase can be compensated for at thereceiving side.

The pre-equalization of the power of the subcarrier signal can belimited to an upper threshold value.

In case along with the pre-equalization the upper threshold is reachedfor a subcarrier signal, the transmission power of the correspondingsubcarrier signal is fixed at the upper threshold value. Instead offurther raising the transmission power the modulation scheme for saidsubcarrier is adapted.

The adaptation of the modulation scheme of a subcarrier signal can besignaled to the receiving side.

To adapt the modulation scheme of a subcarrier signal, the modulationscheme can be 10 simplified or the subcarrier signal can even not bemodulated at all.

In case in a modulation scheme a subcarrier signal is adapted such thatthe bit rate of the subcarrier signal is reduced, the modulation schemeof at least one other sub-carrier signal can be changed to a morecomplex modulation scheme.

The detection of the frequency channel characteristics can be performedon the basis of received pilot symbols.

According to a further aspect of the present invention a computersoftware program 20 product implementing a method as set forth abovewhen run on a computing device of a transmitter is proposed.

According to a still further aspect of the present invention atransmission diversity device adapt for adjusting the transmissioncharacteristics of subcarriers of a multi-carrier transmission systemand having a plurality of antenna elements is proposed. The transmissiondiversity device comprises means for detecting the frequency subcarrierchannel characteristics of the multicarrier transmission.

Furthermore, it comprises a means for distributing the power of eachsubcarrier is distributed to the antenna elements by multiplying(weighting) it (by) the complex conjugate of the correspondingsubcarrier frequency channel or [0, 1] value according to the decisionwhich antenna is selected for the corresponding subcarrier.

The equalizer can be furthermore adapted to divide the sub-carriersignals respectively by the sum of the squared magnitude of thefrequency channel characteristics of all antenna elements or by themagnitude of the frequency channel characteristics of the selectedantenna.

The device can furthermore comprise a pre-equalizer with a phasecompensation function for adjusting the phase of the subcarriersrespectively according to the detected frequency channelcharacteristics.

The pre-equalizer can limit the power of the subcarrier to an upperthreshold. The device can be a base station of a wireless transmissionsystem, such as the OFDM system.

Further features, objects and advantages of the present invention willbecome evident for the man skilled in the art when reading the followingdetailed description of embodiments of the present invention taken inconjunction with the figures of the enclosed drawings.

FIG. 1 shows a pre-equalization technique using one antenna,

FIG. 2 shows an antenna selection technique without equalizing,

FIG. 3 shows the maximum ratio combining (MRC) technique in the case oftwo antennas,

FIG. 4 shows an antenna selection technique together with apreequalizing applied to the case of two antennas on the transmittingside,

FIG. 5 shows the maximum ratio combining technique with pre 3 equalizingapplied to the case of two antennas,

FIG. 6A shows an example to explain the function of a MRC preequalizerwith two antennas, and

FIG. 6B shows an example to explain the function of a MRC preequalizerwith two antennas.

For the sake of clarity at first some technical terms will be shortlyexplained: “TX diversity” designates the distribution of the power of atransmission signal on a plurality of antenna elements. The distributionof the power can be achieved f.e. by means of an antenna selection or aMRC (Maximum ratio combining) technique. According to the antennaselection technique, the entire power of the transmission signal issupplied to one antenna element, whereas according to the MRC techniquethe power is distributed according to the channel response, wherein boththe amplitude and/or the phase of the channel response vector can betaken into account.

Following the TX diversity block, according to the invention a“pre-equalizer” can be provided at the transmitter side. Thepre-equalizer can modify subcarriers of a multicarrier transmissionsystem such that they arrive equally (in amplitude and/or phase) at thereceiving side.

Finally on the receiving side an equalizer can be provided to equalizeall received subcarriers in amplitude and/or phase (but of course notthe modulation constellation). With reference to FIG. 1 at first thepre-equalizing technique for one antenna will be explained. An OFDMsymbol vector

X=[X ₀ ,X ₁ ,X ₂, . . . ]

is transmitted by means of an antenna 3 of a transmitter 5 over awireless channel 2 to an antenna 4 of a receiver 6. The channel responseof the channel 2 can be represented as a vector H, wherein each vectorelement is associated with one frequency subcarrier channel:

H,)[h ₀₀ ,h ₀₁ ,h ₀₂, . . . ]

In the shown pre-equalization technique, the channel response vector Horepresenting the channel characteristics for both amplitude (fading) andphase (phase shift) can be detected 11 in a pilot phase by means of aevaluation of the transmitted symbol xH0 (without pre-equalization). Thechannel response vector H₀ is used in a pre-equalizer 1 such that apre-equalized symbol vector x/H₀ is actually transmitted and theoriginal OFDM symbol vector x is thus received at the antenna 4 of thereceiver 6. As shown the pre-equalization is effected based on theknowledge of the channel response vector H₀.

FIG. 2 shows an antenna selection technique in the case of anapplication to two antenna elements 3, 3′. The OFDM symbol vector x isat first supplied to each of the antennas 3, 3′ to be transmitted overtwo different channels H₀, H₁, (references 2, 2′).

The channel response vectors H₀, H₁, of the two channels 2, 2′ arerespectively detected 15 11, 11′ for example by means of an evaluationof pilot symbols transmitted from the receiver 6 to the transmitter 5and the squared magnitude of the channel response vectors |H₀|² and|H₁|² is respectively calculated. Depending on the calculated squaredmagnitude of the channel response vector H₀ and H₁, a decision/selectionunit 7 decides and selects the best suited antenna element 3 or 3′ foreach subcarrier. To achieve this object the decision/selection unit 7outputs for example bit strings 13 and 13′, respectively, which are thenmultiplied in multiplying units 8, 8′ with the input OFDM symbol vectorx. According to this technique the best suited antenna element for eachsubcarrier can be chosen, i.e. the antenna element with the best channelresponse (inverse of the squared magnitude of the channel responsevector) of each subcarrier is chosen.

FIG. 3 shows a maximum ratio combining (MRC) technique applied on a twoantenna transmission system. As shown in FIG. 3 again a channel responsevector H₀ and H₁, is detected ii, 11′ and the complex conjugate 17, 17′of the channel response H₀* and H₁*, respectively is multiplied by aweighting unit 14, 14′ with the OFDM symbol vector x to be transmitted.This weighting unit 14, 14′ for each antenna element 3, 3′ provides fora TX antenna distribution by applying an appropriate weight. The powerof the subcarrier channels is therefore distributed to each antennaelement proportionally to each channel response. The phase of the signalcan also be adjusted at TX antenna elements 3, 3′, so that the phase atall distributed signals from different antenna elements 3, 3′ can meetequally the receiving antenna 4.

According to the present invention an antenna diversity and anpre-equalizing step can be additionally used at the transmitter. Thetransmitter antenna diversity technique includes the above explained TXantenna selection technique and maximum ratio combining (MRC) method.

With reference to FIGS. 4 to 6 now the normalizing of the transmissionpower of each subcarrier will be explained.

FIG. 4 shows an antenna selection technique combined with thepre-equalizing and applied to a two antenna system.

As the sub-units of the transmitter 5 shown in FIG. 4 have already beenexplained with reference to FIGS. 1, 2 and 3, the same reference signsare used in FIG. 4 and reference is made to the cited figures. As shownin FIG. 4, after the selection of a transmission antenna element 3, 3′,each subcarrier channel is divided in a division (pre-equalizing) unit1, 1′ by the channel response vector H0 of the selected channel.

The subcarrier channel can alternatively be divided by the magnitude ofthe selected channel (see reference 16, 16′ in FIG. 4). In this caseonly the power profile of the channel is compensated for. If only thepower profile, but not the phase of the channel is compensated, thephase of the channel can be compensated at the receiver side 6.

FIG. 5 shows a further embodiment of the present invention according towhich the maximum ratio combining (MRC) technique is combined with apre-equalizing and applied to a two antenna 3, 3′ system. In addition tothe system shown in FIG. 3 each channel is pre-equalized 10, 10′. Asshown in FIG. 5 the squared magnitude of each of the channel responsevectors H₀ and H₁ of each subcarrier channel is calculated 13, 13′. Anadding unit 12 then adds up the calculated squared magnitude of allchannel response vectors and supplies the result to furtherpre-equalizing units 10, 10′.

Respectively one further pre-equalizing unit 10, 10′ is provided foreach channel 2, 2′ 5 dividing the pre-equalized sub-carrier channel xHand xH, respectively, by the sum of the squared magnitudes of thechannel response vectors H₀ and H₁.

As the weighting units 14, 14′ use the complex conjugate of the channelresponse vector H₀ and H₁ according to the embodiment of FIG. 5 both theamplitude (fading) 10 and the phase (phase shift) of the respectivechannel is compensated for. Therefore, after the distribution to allantenna elements 3, 3′, the distributed subcarrier signal can bepre-equalized 10, 10′.

A relatively high transmission power which may cause interference can berequired in 15 some subcarrier channels. These subcarrier channels arethose which present a fading dip with all antenna elements 3, 3′. Usinga plurality of antenna elements 3, 3′, the number of subcarrier channelswhich require a high transmission power can be dramatically reducedcompared to a single antenna case.

As a measure to further decrease the interference problem, thetransmission power can be limited by the equalizers 1, 1′ at a certainupper threshold value. In case the equalization step results in atransmission power judging said threshold value, the transmission poweris no longer increased but the modulation scheme is changed to a simpleone in the corresponding subcarrier channels. Therefore, thepre-equalization is only done until the predetermined power thresholdvalue is reached. At this predetermined power threshold level value thetransmission power is limited to prevent interference to increase.Instead of further increasing the transmission power of the sub-carrierchannel, a simpler modulation scheme is used to those subcarrierchannels or these subcarrier channles are not modulated at all toprevent bit errors to be generated. The use of a simpler modulationscheme for some of the subcarrier channels results in a reduced bit ratewhich can be compensated for by changing the modulation scheme in othersub-carriers (which are in better condition) to a more complicated one.

This modulation scheme shift (adaptation) can be cited for example atthe transmitter (base station) side and then signaled to the other side(for example mobile station).

The channel responses can particularly be estimated from received pilotsymbols.

The main advantage of the present invention is that the error rate canbe reduced for example by means of the pre-equalization. At the sametime the transmission power can be reduced by a combination ofpre-equalization or antenna selection or MRC technique. By means of anadaptive modulation technique the transmission power can be furtherlimited. Therefore, the capacity of a network can be increased all bykeeping the quality of each communication constant.

1. A method for adjusting transmission characteristics of subcarriers ofa multi-carrier transmission system using a plurality of antennaelements, the method comprising: detecting, in a transmitter, subcarrierfrequency channel characteristics of the subcarriers; and pre-equalizinga power of each subcarrier by dividing the power of each subcarrier byamplitude characteristics of the corresponding subcarrier frequencychannel or all subcarrier frequency channels.